The present invention relates to a method for joining thermoplastic resin molded products, and in particular to a joining method using a melt bonding method with electric resistance wire in which the electric resistance wire interposed between the joining surfaces of two molded products that are to be joined is heated by the application of current.
Examples of conventional methods for joining thermoplastic resin molded products include methods in which the joining surfaces that are to be joined are coated with an adhesive to join the surfaces by adhesion, and methods using ultrasonic waves or vibrations, or high frequency electromagnetic induction or electric resistance wires, and the like.
Of these methods, joining methods that feature the use of adhesives are troublesome and time-consuming (specifically, the application of the adhesive is particularly troublesome, and it takes a long time to dry and hold the adhesive under pressure), and the adhesive strength is relatively low, making such methods virtually unusable for products (particularly mass-produced goods) which require a certain level of strength and air-tightness. Joining methods featuring the use of ultrasonic waves or vibrations, or high frequency induction, not only suffer from exorbitant equipment expenses, but the shape of the final product is considerably limited, making it difficult to ensure a satisfactory degree of freedom in the design of the final product.
It is well known, for example, that an intake manifold is connected to a cylinder head in internal combustion engines to feed intake air to the combustion chamber of each cylinder, and that the intake manifold is equipped with a plurality of pipes (equal to the number of cylinders) connected to the engine cylinders and a so-called surge tank that communicates with the intake air feed source. Since intake manifolds are a considerably large object in the air-intake system, their formation with synthetic resins is now being considered as an alternative to conventional light metals (such as aluminum alloys) in order to make the parts around the engine lighter.
The aforementioned intake manifold is used in an air-intake system with lower temperature conditions than the exhaust system, making the application of synthetic resins (particularly types of synthetic resin that are reinforced with fibers or the like) feasible.
When such intake manifolds are manufactured with a synthetic resin, a thermoplastic resin capable of stable strength and rigidity up to a certain temperature is used as the material to form upper and lower halves, which are joined by vibration welding.
More compact engine compartments have led to increasing demand for more compact intake manifolds and mounting structures while ensuring that the pipes are a certain necessary length and that they are designed as much as possible with an equivalent length to ensure good intake characteristics. For that purpose, the pipe shape has developed from relatively linear pipes into bent pipes that have been bent at a predetermined curvature as well as into more complex shaped pipes such as three-dimensionally bent shapes that are not just simple curves but that also have torsion applied thereto. More compact surge tanks have also led to demand to concentrate the connections between the plurality of pipes and the surge tank in as narrow a space as possible, which has also resulted in the need for pipes with more complex shapes.
That is, it is essential to ensure an adequate degree of freedom in the design of the shape of manufactured parts in these cases.
The use of a joining method based on welding with electric resistance wire to exploit the thermal energy produced by the electrification of the electric resistance wire is preferred as an alternative to the aforementioned vibration welding in order to ensure a certain level of bonding strength and air-tightness as well as better productivity during mass production and also to improve the degree of freedom in the design of the shapes of manufactured products in cases where intake manifolds are produced with synthetic resin.
Such a joining method based on melt bonding with electrical resistance wire is described in greater detail, for example, in Japanese Patent Laid-Open Publications 58-59050, 59-118426, 62-267125, 62-288029, 06-64043, 10-16061, and 10-44246, where electric resistance wire is electrified while pressed between the joining surfaces of components that are to be joined, so that the resin around the electric resistance wire is fused by thermal energy and bonded under pressure, the shape of the electric resistance wire being formed to conform to the shape of the surfaces being joined so that products with more complex shapes can be accommodated relatively easily.
The joining methods based on melt bonding with electrical resistance wire disclosed in the aforementioned publications all feature the use of resin materials which can be melted relatively easily, but since they are carried out until final pressurization while the wire is electrified, it is generally difficult to establish electrification and pressurization conditions, making these methods difficult to use without modification, particularly with materials which have a certain level of high strength and high air-tightness and which are difficult to melt.
That is, problems which occur are that, depending on the electrification and pressurization conditions, the resin of the joined parts can be difficult to melt adequately, or conversely the resin can decompose as a result of overheating, or unwanted gases can be produced, making it difficult to consistently obtain high strength and air-tightness characteristics in the parts that are joined.
In view of the foregoing technical drawbacks, an object of the present invention is to provide a method for joining thermoplastic resin molded products together, wherein the resin of the joined parts can be suitably melted so as to ensure satisfactorily high strength and air-tightness in the joined parts when thermoplastic resin molded components are joined together using the melt bonding method with electric resistance wire.
According to a first aspect of the present invention, there is provided a method for joining thermoplastic resin molded products together by pressing together the joining surfaces of two molded products that are to be joined, with electric resistance wire therebetween, when such thermoplastic resin molded products are to be joined together, and by applying current to heat the electric resistance wire, so that the resin around the electric resistance wire is melted and bonded under pressure to join the two molded products, wherein the method for joining thermoplastic resin molded products is characterized by comprising: a first step in which electric resistance wire is sandwiched between the joining surfaces of two molded products which are to be joined, pressure is applied at a first predetermined pressure, and current of a predetermined value is applied for a predetermined time to the electric resistance wire, so that the resin around the electric resistance wire is melted by the thermal energy produced in the electric resistance wire while the joining surfaces of the two molded products are held apart at a predetermined interval; and a second step in which, following the conclusion of the first step, the current being applied to the electric resistance wire is stopped, and a second predetermined pressure is applied to join the molten resin under pressure.
In the method for joining thermoplastic molded products according to the first aspect of the present invention, the resin around the electric resistance wire is melted by thermal energy that is produced in the electric resistance wire while the joining surfaces of the two molded products that are to be joined together are held a certain distance apart in a first stage in which the electric resistance wire is sandwiched between the surfaces of the two aforementioned molded products that are to be joined, and current of a predetermined value is applied for a predetermined time to the electric resistance wire as a first predetermined pressure is applied, allowing a satisfactory molten state of resin to be obtained in a reliable manner by ensuring that thermal energy is provided to the resin around the electric resistance wire without exorbitant increases in the temperature of the electric resistance wire (that is, without causing the material resin to decompose or producing unwanted gas). Upon the conclusion of this first step, a second step in carried out, in which the current to the aforementioned electric resistance wire is terminated, and a second predetermined pressure is applied to join the molten resin under pressure, allowing the resin of the joined portions to be joined under pressure in a reliable manner.
That is, according to the method of the first aspect of the present invention, two stage pressurization is carried out in which the action of the pressure between the two joining surfaces is divided between first and second steps, so that the electric resistance wire can be electrified and heated while the joining surfaces are held apart a certain distance in the first step, and the surfaces that are to be joined together can then be joined under pressure to the final joining position in the second step in order to ensure that the two molded components are joined together under pressure. Consequently, the resin of the joined portions can be suitably melted and joined under pressure in a more reliable manner, and the joined portions where the molded products being joined together can be provided with more satisfactory high bonding strength and air-tightness than when the joining surfaces are joined together under pressure in the final joining position in a one-stroke pressurization step (single stage pressurization) while the electric resistance wire is electrified and heated, as in the past.
Also, according to a second aspect of the present invention, based on the above invention, gapping means for holding at a predetermined interval the joining surfaces of the two molded products is provided in the first step only.
In this case, since, in particular, a gapping means for holding apart at a certain distance the joining surfaces of the two aforementioned molded products is provided only in the aforementioned first step, the aforementioned joining surfaces can be held a certain distance apart from each other in a reliable manner in the first step.
Further, according to a third aspect of the present invention, based on the above invention, the gapping means is a spacer of predetermined thickness, the spacer being interposed between the two molded products in the first step, and being removed from between the molded products in the second step.
In this case, since, in particular, the aforementioned gapping means is, specifically, a spacer of predetermined thickness, and since the spacer is interposed between the two molded products in the aforementioned first step, and is then removed from between the aforementioned molded products in the aforementioned second step, the aforementioned joining surfaces can be held apart a certain distance in a more reliable manner in the first step.
Furthermore, according to a fourth aspect of the present invention, based on the above invention, the gapping means is unified with at least one of the two molded products, the gapping means being deformed by the action of the second predetermined pressure in the second step so as to narrow the space between the joining surfaces of the two molded products that are to be joined.
In this case, in particular, the aforementioned gapping means is formed in a unified manner with at least one of the two aforementioned molded products, and this gapping means is deformed by the action of the second predetermined pressure in the aforementioned second step, so that the space between the joining surfaces of the two aforementioned molded products that are to be joined is narrowed, making it unnecessary to set up a gapping means separately from the molded product, and also eliminating the trouble of removing the gapping means following the conclusion of the first step.
Furthermore, according to a fifth aspect of the present invention, based on the above invention, either of the molded products has a hole in at least the joining surface that is to be joined, and the other molded product has an annular junction that can be fitted to the hole.
In this case, in particular, one of the aforementioned molded products has a hole in at least the joining surface that is to be joined, and the other molded product has an annular junction that is fitted to the aforementioned hole, making it possible to join the two molded products together in a relatively easier and more reliable manner.
Furthermore, according to a sixth aspect of the present invention, based on the above invention, either of the molded products is a hollow element having an opening in at least the joining surface that is to be joined, and the other molded product is a tubular element with a connecting portion that can be joined to the opening.
In this case, in particular, one of the aforementioned molded products is a hollow element having an opening in at least the joining surface that is to be jointed, and the other molded product is a tubular element having a connecting potion that is joined to the aforementioned opening, allowing the portion where the molded products are joined to be provided with satisfactorily high bonding strength and air-tightness when the hollow element and tubular element are joined.